The present invention generally relates to a process for producing a positive electrode for lithium secondary batteries, particularly those with an active material composed of a lithium manganese oxide having a spinel structure.
Lithium-manganese spinels are presently advantageously used as positive electrode material in electrochemical cells of the Li/Li.sub.x Mn.sub.2 O.sub.4 type. The charge/discharge mechanism of such cells is based on the capacity of the manganese oxide to reversibly intercalate and deintercalate active Li.sup.+ ions in its spinel lattice, which forms an open skeletal structure. Consequently, x will vary in parallel with progressive charging or discharging.
To prepare a LiMn.sub.2 O.sub.4 material from MnO.sub.2, one usually begins with a commercially available manganese dioxide which is first thoroughly ground in a mortar with a solid lithium compound, such as lithium hydroxide, lithium carbonate or lithium iodide, and which is then heated for the purpose of sintering. The sinter cake may optionally be comminuted and provided with a conductive agent (graphite) and a binder before it is press-molded to form the finished cathode pellet.
The commercially available manganese oxides (manganese blacks) form a suitable starting material for the preparation of lithium-manganese spinels insofar as they have for a long time been used in the battery industry, their electrochemical behavior has been thoroughly researched, they are an inexpensive raw material and they are environmentally compatible. However, such cathode substances, produced in the manner described, have to date continued to suffer from a lack of adequate cycling stability. The dischargeable capacities of such substances decrease noticeably in the course of their cyclic service life, in particular after a prior high-temperature aging.
Various measures have been proposed for stabilizing the host structure of a manganese spinel. To this end, it has been attempted to substitute some of the manganese in the lattice with foreign metals including Co, Ni and Fe. Japanese Published Specification 4-169065 suggests the production of a positive electrode material for lithium batteries from MnO.sub.2 and lithium salts of formic acid and acetic acid, which are mixed with the manganese oxide (instead of LiOH.multidot.H.sub.2 O or Li.sub.2 CO.sub.3). The mixture is then heat-treated at relatively low temperatures of between 200.degree. C. and 500.degree. C. However, in each case, an unqualified success has not resulted.